The soldier, who can only be known as Corporal Danny for security reasons, shot at the feet of an unarmed man who was apparently directing gunfire at Cpl Danny's unit, an Army spokesman said.
Armed with a .338-calibre rifle, Cpl Danny, from 4th Battalion The Rifles, serving with the 3rd Battalion The Rifles Battle Group, was watching over his patrol in the village of Sadul Kariz, which lies to the north east of Sangin, northern Helmand.
The soldiers stopped to drink tea at a mosque when Cpl Danny saw movement in a compound which had previously been used as an insurgent firing point.
Cpl Danny warned his patrol over the radio, but as the soldiers moved away there was a burst of gunfire.
Cpl Danny could not see exactly where the shots were coming from but he saw a man near the compound pointing towards the patrol.
Uncertain that the man was indeed part of the attack, Cpl Danny took a split-second decision and fired a single shot into the ground five metres in front of him.
At this point, the man ran away and the firing stopped.
Cpl Danny said:
"I knew I had to fire to stop someone in the patrol being killed. He was very lucky I didn't go straight to lethal."
Patrol commander, Lieutenant Charlie Winstanley, said:
"We'll go back for the gunman another time."
Lieutenant Colonel David Wakefield, spokesman for Task Force Helmand, said:
"This is a really good example of what we mean by 'courageous restraint' in the way we conduct counter-insurgency operations.
"A young soldier making a split-second decision but understanding that in counter-insurgency we kill only when we have to, not because we can."
The incident happened in late October.